The significance of engulfment in the process of turbulent entrainment by plumes

The significance of engulfment in the process of turbulent entrainment by plumes H. C. Burridge 1,2 , D. A. Parker 1 , E. S. Kruger 1 , J. L. Partridge 1 and P. F. Linden 1 Department of Applied Mathematics and Theoretical Physics, University of Cambridge. Department of Civil and Environmental Engineering, Imperial College London. hcb39@cam.ac.uk Abstract We present simultaneous two-dimensional velocity and scalar concentrations measured on a central vertical plane in a turbulent plume. We use an edge-detection algorithm to determine the scalar plume edge, and compare the data obtained in both a fixed Eulerian frame and relative to local plume coordinates defined in terms of the instantaneous plume edge. We observe that the time-averaged distributions of vertical and horizontal velocity are self-similar. Measurements in the plume coordinate show that there is significant vertical flow outside the edge of the plume, contrary to the classical notion of purely horizontal velocities at the plume edge. We observe large variations in the plume width and reason these to be due to the passage of large coherent eddies. The flow within the plume and in the nearby ambient fluid depends on whether an eddy is present or absent. When an eddy is present and the plume is wide, the vertical velocities near the plume edge are small, but in regions where the plume is narrow and there is no eddy, large vertical velocities are observed outside the plume. Introduction Turbulent plumes are of significance to the environment and the economy. For example, in 2010 plumes produced by the eruption of the Icelandic volcano Eyjafjallaj¨okull and the Deepwater Horizon oil leak in the Gulf of Mexico had huge environmental impacts and very large economic consequences. The impact of such events and the ultimate fate of the plume fluid, often containing pollutants or contaminants, is largely determined by turbulent entrainment into the plume. The focus of this study is to examine the mechanisms responsible for turbulent entrainment through an experimental investigation of saline plumes in a freshwater environment. We consider a pure plume produced by a steady localised source of buoyancy within a quiescent environment of uniform density. Plumes induce a flow within the surrounding ambient fluid that is typically considered (e.g. Morton et al., 1956) to be predominately horizontal. By making simultaneous measurements of the flow velocities (using particle image velocimetry, PIV) and the scalar edge (using light induced fluorescence, LIF) of high P´eclet number saline plumes we show that this is not the case. Indeed, we show that vertical velocities at, and outside, the edge of the plume are significant, and that entrainment of ambient fluid is caused by engulfment by large scale eddies. Experiments The experiments were performed in a glass tank of horizontal cross-section 100 cm x 80 cm filled with dilute saline solution (of uniform density ρ a ) to a depth of 85 cm. Relatively VIII th Int. Symp. on Stratified Flows, San Diego, USA, Aug. 29 - Sept. 1, 2016